Based on a growing body of data, it has been postulated that levels of specific intratumoral enzymes influence the action of particular chemotherapeutic agents. Using polymerase chain reaction (PCR) technology, and a radioligand binding assay, this project will attempt to determine whether the intratumoral level of gene expression and the quantitation of protein for the 5-fluorouracil (5FU) target enzyme thymidylate synthase (TS) influences response to combination chemotherapy, and/or whether these levels change during and after treatment. Intratumoral reduced folate pools will be assayed at the same time as the TS determinations to ascertain if these levels affect response or change during treatment as well. Patients with primary gastric and esophageal cancers will be treated with preoperative chemotherapy. To isolate cisplatin's effect on reduced folate pools, the chemotherapy in each program will employ high dose cisplatin followed in one week by a protracted-infusion of 5FU with leucovorin. For patients with esophageal cancer radiation will be added with the first cycle of protracted-infusion 5FU. Following neoadjuvant therapy an operation will be done to remove the primary cancer. By measuring gene and protein expression of TS and reduced folate pools within esophageal and gastric neoplasms at these intervals this project will define whether levels of intratumoral TS and reduced folate pools affect response to therapy or change with therapy. Such correlations will lead to a better understanding of innate and acquired tumor resistance in these malignancies and enhance rational treatment design. Eventually, neoadjuvant and postoperative adjuvant treatments may be individualized on the basis of whether tumors are innately resistant, acquire resistance or remain sensitive despite this intensive preoperative therapy.